http://www.unodc.org/unodc/en/world_drug_report.htm
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Some 200 million people, or 5 percent of the global population age 15-64, have used illicit drugs at least once in the last 12 months. Among this population are people from almost every country on earth. More people are involved in the production and trafficking of illicit drugs and still more are touched by the devastating social and economic costs of this problem. Partially a consequence of its pervasiveness and partially a consequence of the illicit and hidden nature of the problem, reliable analysis and statistics on the production, trafficking and use of illicit drugs are rare.
The World Drug Report 2006 endeavours to fill this gap. It provides one of the most comprehensive overviews of illicit drug trends at the international level. In addition, it presents a special thematic chapter on cannabis, by far the most widely produced, trafficked and used drug in the world. The analysis of trends, some going back 10 years or more, is presented in Volume 1. Detailed statistics are presented in Volume 2. Taken together, these volumes provide the most up-to-date view of today's illicit drug situation.
When I come to think of it, how do they even know this?